A magnetic storage disk used in the disk drive takes the form of a flat solid substrate with a central opening. The material used for the substrate can be aluminum, glass, ceramic, plastic, or other composite material depending on the application involved. A thin layer of magnetic material is coated on both surfaces of the substrate, followed by a protective layer.
For data reading from or writing onto the magnetic layer of a magnetic storage disk, a transducer read-write head is positioned over each disk surface. This transducer read-write head is usually supported by an overhanging actuator arm and mounted onto an aerodynamically shaped block that glides at an extremely small distance from the disk surface when the disk is spinning. This extremely small distance to the disk surface is required for the transducer read-write head to read or write data patterns into the magnetic layer of the disk. The actuator arm carrying the read-write head is pivoted about an axis so that the read-write head can be swung in and out of the disk surface as desired.
Storage data are laid down along circular tracks around the disk surface. To enable the read-write head to know where it is positioned, servo data is pre-recorded along circular tracks on the disk. These servo data contain information that is readable by the read-write head and subsequently is interpreted by the servo system firmware to determine which track the head is on, and to keep the head on the desired track.
Writing down the tracks of servo data onto the disk surface is an essential step in the manufacturing of a disk drive. Servo track writing is normally carried out in the manufacturing plant. As the servo tracks serve as the positional and timing references for a read-write head, precise positioning and timing is required when the servo track is laid down on the disk surface.
One scheme for writing the servo data is to carry out the writing process after magnetic disks have been placed inside the enclosure of a disk drive. Part of the drive enclosure is temporary left open to expose the actuator arm carrying the read-write head. Additional fixture with precision positioning feedback is then used to guide the movement of the actuator arm over the disk surface. This additional fixture can take the form of an external actuator arm. In one variation, the external actuator arm is made to physically push the drive actuator arm along. In another variation, an edge sensor is used ensure that the drive actuator arm will move in synchronous with the external actual arm.
In the self-servo writing scheme, the disks are partially servo-written in the disk drive using one of the above-mentioned schemes in the cleanroom. The rest of the servo tracks are then filled in after the disk drives are taken outside the cleanroom. The intention is to optimize the usage of the cleanroom space.
The additional fixture can also take the form of a small sheet scale embedded in the drive actuator arm. Using an external reader, the position of the drive actuator arm can be read from the scale and controlled directly. By using the embedded scale for positioning, no external actuator is required. The drive actuator still maintains its lightweight and the system is less affected by external excitation such as air turbulence and vibrations. The system is also more tolerant of the vertical vibration of the actuator arms. This method of positioning the drive actuator arm does not require a temporary opening in the drive enclosure. The servo track writing can therefore be carried out outside the cleanroom. The only drawback is perhaps the increase in manufacturing cost by having a sheet scale to be included in every disk drive.
Other schemes have been devised to enable the servo writing process to be written outside the cleanroom. One method is to use an optical clock head for servo writing. The optical clock head reads a timing sheet scale attached to the rotation disk or spindle by optical means through a transparent window. Using such a transparent window in the controlling of the drive actuator arm, the disks in the disk drives may be servo written outside the cleanroom.
In another servo-writing scheme, the process is carried out before the disks are placed inside the drive enclosure. The disks are servo-written on dedicated equipment, commonly called media servowriter, in the cleanroom. One advantage of this scheme is that multiple disks can be servo-written at any one time using the same set of hardware. As disk drives nowadays usually only contain one to two disks, in-drive servo-writing schemes described in the previous paragraphs can take up a lot of cleanroom space and time. By servo-writing multiple disks outside the drive, usage of cleanroom space and time can be substantially reduced. Using better performance components, equipment dedicated for servo-writing is capable of writing servo-tracks with higher quality. This becomes especially important as the track density of the disk media increases.
On a media servowriter, the disks are stacked and packed in a hub that is attached to the spinning shaft of an air-bearing spindle motor. During servo-track writing, the rotary motion of the spindle shaft spins the hub with its disk pack. In order to ensure that the hub is tightly and centrally held down to the rotating spindle shaft when the servo track is being written, one approach is to use a clamp that only exerts a radially inward force to hold the hub. The evenly distributed force pushed the hub towards the center of the spindle shaft while at the same time hold the hub tight for rotation. This is to ensure hub is clamped tightly during spinning. Hub and hub clamp are machined with precision and well balance, when spindle rotates at high speed the mechanical vibration is minimal. This will ensure read-write on the disk can be performed smoothly. The quality of the servo tracks written will also be compromised.
Furthermore, the disk hub should be designed such that it can be easily detached from the spindle shaft after the servo-writing process, while the hub clamp responsible for holding the hub to the spindle shaft should be so designed that it allows the disk hub to be easily removed. A removable disk hub allows the disks to be stacked and un-stacked outside the media servowriter equipment, enabling more efficient time usage of the media servowriter.